The invention relates generally to an apparatus and method for cancellation of ringing in magnetic resonance. More specifically, the invention relates to an apparatus and method for cancellation of ringing in nuclear quadrupole resonance (NQR) utilizing a composite pulse.
It is known that NQR can be an effective means of detecting materials containing quadrupolar nuclei (such as 14N, 35,37Cl, etc.) that might be concealed in luggage, mail, small cargo or on a person. The detection of nitrogenous or chorine-containing explosives or narcotics utilizing NQR is of particular interest in luggage and passenger screening operations, in which a large quantity of materials or number of persons must be scanned in an efficient and non-invasive manner. As an example, consider luggage scanning where an object to be scanned is typically placed inside a large inductive coil that is part of a tuned resonance circuit. High power radio-frequency (RF) pulses are then applied to the circuit leading to an oscillating magnetic field inside the coil. The magnetic field excites the NQR signal which is subsequently detected as oscillating magnetic field via the same coil.
Unfortunately, the RF pulses utilized in typical NQR detection sequences will induce an acoustic ringing in certain circumstances. Although there are several causes and associated descriptions of acoustic ringing, one major effect is caused by the presence of a metallic material containing permanent magnetic moments within the coil. The applied oscillating magnetic field interacts with the magnetic moments, causing temporary rearrangements in the relative orientation of the magnetic moments. The rearrangement of the magnetic moments can lead to an actual change in the physical dimensions of the object—a phenomenon commonly known at the magnetostrictive effect. Following the RF pulses, the relaxation of the permanent moments leads to an acoustic ringing signal which is detected along with a true NQR signal from the nuclear spins of interest. In some situations, the acoustic ringing signal can be several orders of magnitude larger than the true NQR signal, making observation of the true NQR signal difficult if not impossible. Failure to cancel the acoustic ringing can cause an increased false alarm rate, i.e., an increased number of instances in which a detection threshold is reached due to the acoustic ringing signal instead of the underlying true NQR signal.
A current approach for reducing effects due to acoustic ringing involves a combination of two specific pulse sequences known as PAPS (phase-alternated pulse sequence) and NPAPS (non-phase-alternated pulse sequence), which involve a string of pulses, in between which signal is acquired. This technique, which is described in U.S. Pat. No. 5,365,171 issued to Buess et al. and entitled “Removing the Effects of Acoustic Ringing and Reducing Temperature Effects in the Detection of Explosives by NQR”, the contents of which are herein incorporated by reference, makes use of the fact that the signal of interest has a different phase relationship to the applied RF pulses than the acoustic ringing signal in the two sequences. Combining the results from each sequence allows the suppression of the acoustic ringing signal.
The success of the PAPS and NPAPS approach, however, relies on the acoustic ringing decaying to zero between the applied pulses and leads to limitations in the pulse separation. Furthermore, the ability to observe the final signal is strongly dependent on the relaxation properties of the material under investigation and, in unfavorable cases, no true nuclear signal is observable even in the absence of acoustic ringing.
In view of the above, it would be desirable to provide a detection apparatus that was not susceptible to acoustic ringing and a method of eliminating or canceling acoustic ringing from a detected nuclear resonance signal.